Groundwater and root trait diversity jointly drive plant fine root biomass across arid inland river basin

Background and aims As one of the key determinants of water availability in arid regions, groundwater plays an essential role in regulating biodiversity, key ecosystem processes and functioning. Meanwhile, biodiversity is known to modulate ecosystem functions. Numerous studies have focused on the links between aboveground productivity and plant diversity, while the potential influence of root traits on fine root biomass (FRB) has barely been elucidated. Therefore, it remains unclear how groundwater, soil, species diversity, above- and belowground traits jointly regulate FRB. Methods 230 plots were sampled under a broad range of groundwater and soil conditions across a typical arid inland river basin, within which FRB, associated soil properties, species diversity and functional traits were measured. Results FRB was mainly regulated by the interactions between groundwater and root trait diversity, followed by groundwater, root trait diversity and soil moisture. Although mean and seasonality of groundwater depth had substantial effects on FRB, groundwater affected FRB mainly through indirect pathways mediated by root trait diversity. Furthermore, root trait diversity showed the largest direct effect on FRB, suggesting that it was the main driver of FRB. Species richness and aboveground traits had no significant direct effect on FRB, implying that root traits surpassed species richness and aboveground traits in regulating FRB. Conclusion Our findings provide robust empirical evidence that groundwater and root trait diversity jointly drive existing belowground carbon storage in arid inland river basin. We emphasize the importance of groundwater and root trait diversity, as well as their interactions in forecasting underground ecological processes and functions under global changes.